In a liquid crystal display device, liquid crystal is a substance that does not itself illuminate light. Instead, the liquid crystal relies on reflecting light from a light source, thereby displaying images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
Referring to FIG. 6, a typical bottom-lighting backlight module 10 is shown. The backlight module 10 includes a frame 11, a plurality of optical sheets 13 and a plurality of light sources (not shown). The frame 11 is a rectangular-shaped with four peripheral sidewalls 112. Two holding recesses 112 are respectively defined on a top of two opposite sidewalls 112, and two positioning pins 114 are respectively defined on a bottom surface of each holding recess 112. Each optical sheet 13 is in a similar rectangular shape according to the frame 11. The optical sheet 13 includes two protrusions 132 and two positioning holes 134. The two protrusions 132 are defined in the periphery area of the optical sheet 13 corresponding to the holding recess 112. The two positioning holes 134 are respectively defined in the protrusions 132 according to the positioning pins 114. The optical sheets 13 may be fixed on the top of the frame 11 by mounting the positioning pins 114 in the corresponding positioning holes 134. The light sources (not shown) are disposed under the optical sheet.
However, the structures, i.e. the holding recesses 112 and the positioning pins 114, employed for positioning and holding the optical sheets 13 relative to the frame 11 in the above typical direct type backlight module 10 has the following drawbacks to be solved.
In order to adjust to the deformation of the optical sheets 13, diameters of the two opposite positioning holes 134, are configured to be different, such as one positioning hole 134 having a smaller diameter is used to precisely position the optical sheets 13 by the corresponding positioning pin 114 inserted therethrough, and the other positioning hole 134 having a larger diameter is used to configure a suitable larger space to engage with the corresponding positioning pins 114 when the optical sheets expand and contract. However, the optical sheets 13 may also deformed due to the following other reason: un-uniformity temperature distribution in the backlight module, different variables produced when manufacturing and assembling the backlight module, and so on.
What is needed, therefore, is a direct type backlight module that overcome the above mentioned drawbacks.